1. Field of the Invention
The present invention relates to a motor control apparatus for controlling the angular displacement of an electric motor which selects operated positions of a controlled object.
2. Description of the Related Art
To meet requirements for better space saving capability, improved assemblability, and increased controllability, more and more mechanical systems on automobiles and other equipment have in recent years been replaced with motor-operated systems. For example, Japanese laid-open patent publication No. 2002-323127 discloses a motor-operated gear range selector mechanism for an automatic transmission for use on vehicles. The disclosed motor-operated gear range selector mechanism has an output shaft connected to the rotational shaft of a motor through a speed reducer. The gear range selector mechanism is operated by the output shaft for selecting gear ranges of the automatic transmission. The motor is combined with a rotational angle detector, i.e., an encoder, for detecting the angular displacement or rotational angle of the motor. For selecting a desired gear range, the motor is energized to rotate the rotational shaft to a target position (target count) corresponding to the desired gear range based on the count of output pulses from the encoder, thereby shifting the automatic transmission into the desired gear range.
The angular displacement or rotational angle of a motor which controls an object is converted into the angular displacement of the controlled object, i.e., the controlled variable of a gear range selector mechanism, through a rotation transmitting system such as a speed reducer or the like. The rotation transmitting system is made up of various discrete components, and there is play or lost motion between those components. For example, there is play or backlash between the gears of a speed reducer mechanism. If a joint having a noncircular cross section, such as a rectangular cross section, a D-shaped cross section, or the like, on the tip end of the rotational shaft of the speed reducer mechanism is fitted into a hole in the joint shaft of the controlled object, then a clearance needs to be provided between the joint and the inner surface of the hole to allow the joint to be easily fitted into the hole. Therefore, since the rotation transmitting system for transmitting the angular displacement of the motor into the angular displacement of the controlled object has play or lost motion, even when the angular displacement of the motor is accurately controlled based on the angular displacement detected by the rotational angle detector, the resultant angular displacment placement of the joint shaft of the controlled object, i.e., the resultant controlled variable of the gear range selector mechanism, suffers an error commensurate with the play or lost motion of the rotation transmitting system. Consequently, the controlled variable of the gear range selector mechanism cannot be controlled with accuracy.
In view of the above drawback, there has been proposed a technology for energizing the motor under bump control until the movable element of the gear range selector mechanism bumps into a limit position of the movable range thereof, i.e., a wall of the gear range selector mechanism, thereby learning the amount of play included in the rotation transmitting system, and setting or correcting a target position in view of the learned amount of play, as disclosed in Japanese laid-open patent publication No. 2004-23932.
According to the disclosed proposal, after the movable element of the gear range selector mechanism bumps into a limit position of the movable range thereof, i.e., a wall of the gear range selector mechanism under bump control, the movable element is also caused under bump control to bump into another limit position of the movable range, i.e., another wall of the gear range selector mechanism. Pulses that are output from the encoder while the motor is being energized from one of the limit positions to the other are counted to measure the movable range of the gear range selector mechanism. The difference between the measured movable range of the gear range selector mechanism and the designed movable range of the gear range selector mechanism is learned as the amount of play in the rotation transmitting system.
The gear range selector mechanism disclosed in Japanese laid-open patent publication No. 2004-23932 has two gear ranges, i.e., a parking range and a non-parking range. When the gear range selector mechanism is to shift the transmission into the parking range or the non-parking range after the engine has started, bump control in the parking range and bump control in the non-parking range are successively performed to learn the amount of play in the rotation transmitting system.
If, however, the gear range selector mechanism is to shift the transmission into four ranges, i.e., a parking range (P range), a reverse range (R range), a neutral range (N range, and a drive range (D range), then for learning the amount of play in the rotation transmitting system, bump control in the P range is performed, and thereafter bump control in the D range needs to be performed when the transmission is shifted successively into the P, R, N, and D ranges. While the gear range selector mechanism shifts the transmission between the P, R, and N ranges, bump control in the D range cannot be performed. This is because if the transmission is automatically shifted into the D range against the will of the driver while the engine is in operation, then the vehicle may get into motion undesirably. Consequently, until the driver moves the gear shift lever into the D range, the amount of play in the rotation transmitting system cannot be learned, and the controlled variable of he gear range selector mechanism suffers an error commensurate with the play or lost motion of the rotation transmitting system. As a result, the gear range selector mechanism may possibly fail to select desired gear ranges based on the gear shift lever operated by the driver.